This invention relates to improvements surge voltage arresters for line protectors of the type used for protecting telephone lines and like communication lines from over-voltage and over-current conditions.
Surge voltage arrester of the cold cathode gas discharge tube type serve as the primary arrester and source of protection in various line protectors of the station or central office type. Such line protectors may also include a carbon or other type of air gap back-up protector in the event of a failure of the primary surge arrester as a result of leakage of gas from the tube due to a broken seal or similar damage. A gas tube arrester which has failed in this manner will be difficult to detect because the line to which it is connected continues to operate properly. Thus, it is desirable to provide some type of air gap or secondary surge arrester as a "back-up" or ventsafe feature in the event of failure of the gas tube arrester. Line protectors embodying these surge voltage arresters are frequently installed under conditions wherein dust, moisture and other contaminants can enter the secondary air gap. This can alter the breakdown voltage characteristics of the air gap.
In providing secondary or back-up protection of the air gap type the ideal situation is to construct the air gap with a breakdown surge voltage that is slightly above the breakdown surge voltage of the gas tube arrester. In this way the secondary air gap is not utilized so long as the gas tube is functioning properly. However, the idealized situation is not attainable on a mass production basis. Thus, there is always a range over which the gas tube breaks down, and this will depend upon many factors, including production tolerances as well as the number of times the gas tube has fired. Likewise, as far as the air gap is concerned, production tolerances, electrode surface conditions, and other factors will result in a variation of breakdown voltages from unit to unit.
Where the air gap is made quite small in order to provide a low breakdown voltage, there is the possibility that the breakdown voltage of the air gap may in some cases be below that of the gas tube, in which event the air gap would break down while the gas tube is still functioning properly. Moreover, in a typical arrangement in which the electrodes of the air gap are of metal, the air gap will short out after one or a few discharges, leaving a surge arrester unit that has short-circuited the line but which nevertheless has a properly functioning gas tube. On the other hand, if the air gap is made large in an attempt to prevent short circuitry, its breakdown voltage may be so high that it exceeds the specifications or requirement of the user. Therefore, in surge arresters having facing metal surfaces that define the secondary arc gap a compromise has been attempted so as to provide an arc gap which is small enough to break down at a low enough voltage for useful purposes, but which breakdown voltage is nevertheless above the breakdown voltage of the gas tube.